Process for the purification of sulfur

ABSTRACT

A process for the purification of sulfur that contains impurities is described. In a contact zone ( 1 ), solid sulfur ( 2 ) is brought into contact with at least one organic solvent ( 16 ) that is selected from the group that is formed by monohydric alcohols with 8 to 40 carbon atoms, polyols that comprise 2 to 8 hydroxyl groups and that have 8 to 40 carbon atoms, polyalkylene glycols and polyalkylene glycol ethers under suitable conditions for carrying out the melting of sulfur. The supply of sulfur is halted, and the mixture that is obtained is allowed to decant so as to produce the segregation or liquid/liquid separation of the purified liquid sulfur that is obtained and organic solvent that contains impurities. At least a portion of the purified liquid sulfur is drawn off ( 8 ) from the lower portion of the contact zone. The solvent can be used until it is saturated with products that are dissolved and/or in suspension, no longer allowing the separation of the liquid sulfur and the solvent. It can be either regenerated and recycled or eliminated.  
     Application of the process for the purification of the sulfur that is obtained in a “redox” process.

[0001] This invention relates to a discontinuous process for thepurification of sulfur by washing with an organic solvent. The sulfurthat is to be purified is actually contaminated by solid or liquid,organic or inorganic chemical radicals. The process according to theinvention pertains to, for example, the sulfur that is obtained in a“redox” process for desulfurization of a gas that contains at leasthydrogen sulfide. This type of process can use a catalytic solution thatcomprises at least one multivalent metal (Fe³⁺ or V⁵⁺, for example) thatmay or may not be chelated by at least one chelating agent undersuitable conditions for carrying out the oxidation of the hydrogensulfide into elementary sulfur and the simultaneous reduction of themultivalent metal that may or may not be chelated from a higher degreeof oxidation to a lower degree of oxidation. The gaseous effluent thatis recovered is almost free of hydrogen sulfide. The catalytic solutionis at least partially reduced and contains elementary sulfur. Sulfur canbe removed from the catalytic solution before or after the regenerationstage of the catalytic solution. The sulfur that is collected at leastin part is then purified according to the process of the invention.

[0002] Patents FR-A-2 700 713 and U.S. Pat. No. 4,664,902 illustrate thetechnological background.

[0003] The prior art describes numerous “redox” processes and relateddevices that make it possible to eliminate the hydrogen sulfide and torecover the elementary sulfur that is formed during the process.

[0004] By way of example, a desulfurization process that uses an ironchelate comprises, for example, the two oxidation-reduction stagesbelow:

[0005] In a first stage (absorption of gas and oxidation-reductionreaction), the hydrogen sulfide that is present in the gas that is to betreated reacts with, for example, chelated ferric ions according to theinvention:

H₂S+2Fe³⁺(chel.)→S+2H⁺+2 Fe²⁺(chel.)  (1)

[0006]  in a second stage (regeneration), the ferrous ions arereoxidized by the oxygen of the air following the reaction:

2 Fe²⁺(chel.)+2 H⁺+½O₂→2 Fe³⁺(chel.)+H₂O  (2)

[0007] The catalytic solution may or may not be an aqueous solution ofchelated iron, produced from ferrous or ferric iron salts such assulfates, nitrates, thiosulfates, chlorides, acetates, oxalates and/orphosphates. The ferrous and ferric ions can be replaced respectively byvanadate ions IV and V. The catalytic solution can also contain sodium,potassium, and ammonium ions, carbonates, and/or anthraquinonedisulfonates.

[0008] The chelating agents, used individually or in a mixture, can beorganic compounds that are known for their complexing properties, forexample acetylacetone, citric acid, salicylic acid, sulfosalicylic acid,tiron (catechodisulfonic acid), dimercapto-2,3-propanol and amino acids,such as, for example, EDTA (ethylenediamine tetraacetic acid), HEDTA(hydroxy-2-ethylenediamine triacetic acid), NTA (nitrilotriacetic acid),DCTA (diamino-1,2-cyclohexane tetraacetic acid), DPTA(diethylenetriamine pentaacetic acid), IDA (iminodiacetic acid) and ADA(N-(2-acetamido)iminodiacetic) acid).

[0009] The solid sulfur that is formed in the “redox” processes is inclose contact with the catalytic solution. Most often, the sulfurrecovery techniques that are used are mechanical: filtration, flotationor centrifuging. The amount of catalytic solution that is entrainedphysically with the sulfur is therefore significant. Consequently, thesulfur that is produced is of poor quality.

[0010] In contrast, during second stage (2), secondary reactions canarise, in particular the degradation of the chelating agent, which causethe formation of products that accumulate in the catalytic solution andthat can precipitate. These products, organic or inorganic compounds,are also entrained with the sulfur and contribute to its poor quality.

[0011] Thus the sulfur, obtained in particular from “redox” processes,is not of adequate quality, in particular in the hypothesis of anapplication in the chemistry sector.

[0012] The prior art describes processes and devices that make itpossible to purify the elementary sulfur that is formed in “redox”processes.

[0013] U.S. Pat No. 5,122,351 describes a method where the sulfur iswashed by suspending it in water, then it is melted to be separated fromthe aqueous solution. The washing solution is concentrated byevaporation and reinjected in the “redox” process in the H₂S absorptionstage. The evaporated water is recycled for washing the sulfur. Theinvestment costs of such a process are significant. Among other things,a pressurized separator and an evaporator are required. In addition,this process imposes the use of costly materials for limiting thecorrosion problems that are linked to the presence of water that areincreased at high temperature. Furthermore, the water-insoluble productswill not be eliminated.

[0014] U.S. Pat. No. 4,705,676 describes a method for purifying sulfurby melting sulfur in a phase separator under an inert atmosphere. Thesupernatant aqueous catalytic solution is reinjected in the absorber orin the regenerator. The liquid sulfur is filtered. This process requiresa pressurized separator that can resist corrosion. In addition, thefiltration of the liquid sulfur can be considered only with a large-porefilter to limit the clogging problems. Fine particles thus will remainmixed with the sulfur.

[0015] U.S. Pat. No. 4,517,170 describes a method for extraction ofsulfur from the catalytic solution of a “redox” process by suspendingsolid sulfur in a mixture of aliphatic hydrocarbons that have 4 to 8carbon atoms and recovery of the aqueous catalytic solution. The sulfurand the hydrocarbons can then be separated following different methods.If the solid sulfur is separated mechanically (filtration, centrifuging,. . . ), it will contain a significant amount of hydrocarbons thatlimits its purity.

[0016] According to a variant of this method, the sulfur suspension inthe hydrocarbons can be heated in a separator to a sufficienttemperature to allow the melting of the sulfur. The use of a pressurizedseparator to keep the hydrocarbons in liquid phase and to obtain a phaseseparation produces high costs. In addition, the sulfur that is thusobtained has a purity that is reduced by the solubility of thehydrocarbons in the liquid sulfur.

[0017] Finally, if the separator is at a pressure that is close toatmospheric pressure, the hydrocarbons are vaporized. This vaporizationleads to residues that pollute the sulfur. This process therefore leadsto sulfur of a reduced purity.

[0018] The object of the invention is to propose a new method forwashing sulfur that offers in particular the advantage of resulting in asulfur of very high purity. Actually, according to the invention, thesulfur is washed in the liquid state, which makes it possible toeliminate any product that would remain adsorbed on the sulfur in thesolid state. In addition, the invention uses an organic solvent that isalmost insoluble in the liquid sulfur and that makes it possible toeliminate any compound that is soluble or insoluble in the solutionsthat are now used in the “redox” processes. The finest particles arealso eliminated by extraction in the washing solvent. The process thatis described in the invention furthermore has lower investment coststhan the above-mentioned processes of the prior art. Actually, thechamber for bringing the sulfur into contact with the solvent is used ata pressure that is close to atmospheric pressure. In addition, it may bemade of carbon steel because washing the sulfur does not require anyaddition of water, and water, optionally present in the sulfur, isvaporized under suitable operating conditions.

[0019] Finally, the purification of the sulfur as it is carried out bythis invention makes it possible, on the one hand, to upgrade it, and,on the other hand, to reduce the volumes for disposal in the case wherethe unpurified sulfur would be considered as waste.

[0020] This invention relates to a process for the purification ofsulfur that is obtained from, for example, a “redox” process and thatcontains impurities.

[0021] It is characterized in that it comprises the series of thefollowing stages:

[0022] In a contact zone, solid sulfur is brought into contact with atleast one organic solvent that is selected from the group that is formedby monohydric alcohols with 8 to 40 carbon atoms, whereby the polyolscomprise 2 to 8 hydroxyl groups and have 8 to 40 carbon atoms,polyalkylene glycols and polyalkylene glycol ethers under suitableconditions for carrying out the melting of sulfur.

[0023] The supply of sulfur is halted, and the mixture that is obtainedis allowed to decant so as to carry out the segregation or theliquid/liquid separation of the purified liquid sulfur that is obtainedand the organic solvent that contains impurities.

[0024] At least a portion of the purified liquid sulfur is drawn offfrom the lower portion of the contact zone.

[0025] The solid sulfur that is to be treated is generally brought intocontact with the organic solvent under suitable heating conditions tocarry out the melting of sulfur.

[0026] The products that are contained in the sulfur are separated afterdecanting. They can be, for example, vaporized or dissolved in thesolvent or can also be concentrated at the interface between the liquidsulfur and the solvent, preferably in the solvent.

[0027] The liquid sulfur that is thus washed is recovered. It is pureenough to make it upgradable.

[0028] The organic solvent can be used until it is saturated withproducts that are dissolved and/or in suspension, no longer allowing theseparation of the liquid sulfur and the solvent.

[0029] The organic phase can then be incinerated or disposed of at leastin part.

[0030] According to a variant of the invention, the solvent may be atleast partly regenerated. More specifically, it is possible to draw offat least a portion of the organic solvent that results from thedecanting stage that contains impurities; said organic solvent ispurified so as to remove from it at least a portion of the impurities;at least a portion of the purified organic solvent is collected, and itis recycled in the contact zone. According to an implementation of theprocess, the solid products are separated from the solvent according toa treatment method or methods such as those described in Patent FR2784370.

[0031] The washing solvent is at least one organic solvent that can beselected from the group that is formed by:

[0032] the heavy alcohols that have 8 to 40 carbon atoms,

[0033] the polyols that comprise 2 to 4 hydroxyl groups and that have 8to 40 carbon atoms,

[0034] the polyalkylene glycols,

[0035] the ethers of these polyalkylene glycols.

[0036] These heavy alcohols and polyols can be linear or branched. Theheavy alcohols can be primary, secondary or tertiary, preferably primaryfor cost reasons.

[0037] Organic solvents that can be used according to the invention arecited by way of examples:

[0038] 1-Octanol, 1-dodecanol, 1-hexadecanol, 9-heptadecanol,1-cicosanol, 1,4-heptadecanediol, 1,4,8-pentadecanetriol, pentaethyleneglycol, hexaethylene glycol, octaethylene glycol, diethylene glycol,triethylene glycol, polyethylene glycol 400 (with a mean molar mass byweight of 400 g/mol), tripropylene glycol, propylene glycol,polypropylene glycol 600 (with a mean molar mass by weight of 600g/mol), octaethylene glycol ethyl monoether, terapropylene glycol butylmonoether, polyethylene glycol 400 methyl monoether, tetrapropyleneglycol ethyl monoether. It is preferably possible to use a polyethyleneglycol due to its insolubility in the liquid sulfur, its low vaporpressure and its good thermal stability.

[0039] Other advantages and characteristics according to the inventionwill be better understood from reading the description below of theembodiment that is described by way of nonlimiting example by referringto FIG. 1 that shows a diagram of an arrangement of the necessaryequipment for the implementation of the process according to theinvention in the case of the regeneration and recycling of solvent.

[0040] The device for implementing the process by batch according to theinvention, which is described in FIG. 1, comprises a chamber forbringing into contact (1) the sulfur to be treated with the solvent,where the melting of sulfur, its washing preferably while being stirredmechanically and the segregation or liquid/liquid separation of theliquid sulfur and the solvent are carried out. The sulfur that is to betreated may contain 2 to 90% by weight of a “redox” catalytic solution.Subjecting sulfur to a prior separation stage (filtration, centrifuging,. . . ) advantageously makes it possible to recover a portion of thecatalytic solution. The sulfur that is to be treated preferably contains5 to 50% by weight of catalytic solution.

[0041] The temperature of the chamber for contact with the washingsolvent is usually between 120 and 160° C., preferably between 125 and150° C., thanks to conventional heating means. The sulfur is generallymixed with the solvent while being stirred in suitable proportions: 5 to80% by weight of sulfur to be treated and preferably 40 to 60% byweight.

[0042] The sulfur-solvent mixture is preferably carried out while beingstirred. After the introduction of the sulfur is stopped, the stirringcan be stopped or maintained preferably from 5 to 60 minutes. The waterand/or organic products that are optionally contained in the sulfurvaporize.

[0043] The stirring is then halted, and after decanting, three zones canbe distinguished in the chamber. The decanting lasts from 1 to 60minutes and preferably 2 to 15 minutes.

[0044] The first zone, upper, is the organic phase: the washing solventthat can contain impurities in the form of products that are dissolvedand/or in suspension. The cover located above the organic phase can bean inert gas.

[0045] The second zone is the interface between the liquid sulfur andthe solvent, a zone that contains the most solid products as impurities.

[0046] Finally, the third zone is the bottom of the chamber thatcontains only liquid sulfur.

[0047] Contact chamber (1) is equipped with a mechanical stirrer (17), apipe for feeding (2) solid sulfur that is to be purified (for example aheated hopper that is equipped with a pipe (3) that allows theintroduction of sulfur), a pipe for evacuating vaporized compounds (4),a pipe for drawing off (9) solvent that is to be recycled and a pipe forfeeding (16) solvent, recycled after the elimination of impurities, inthe upper portion of the chamber.

[0048] At the outlet of chamber (1), the vaporized products are fed viapipe (4) to, for example, a heat exchanger (5), where the water and/ororganic products that are optionally contained in the sulfur arecondensed. The residual gaseous effluent can be sent via pipe (6) to anactivated carbon (7) before disposal.

[0049] The purified liquid sulfur is recovered via a pipe (8) at thelower end of chamber (1).

[0050] The solvent can be recycled. When the quality of the sulfur is nolonger satisfactory, all of the solvent or the solvent that contains themajority of the impurities can be taken up by a pipe (9) in the lowerportion of chamber (1) through a valve (10) via a pump (11) and can besent into a system for purification of solvent (12). It can involve, forexample, a system for filtration of solid products or a system forwashing with water in proportions that allow the solubilization of solidproducts and the separation of the aqueous phase and the organic phase.At the outlet, the purified solvent is recycled via a pipe (13) througha valve (14) via a pump (15) to chamber (1) for treatment of sulfur. Anaddition of solvent via a line (18) can be carried out upstream frompump (15). Elements (9) to (16) are preferably heated to a suitabletemperature to prevent the crystallization of sulfur. The solvent can beused until the phase separation is no longer defined.

[0051] The example that is given below has as its objective to make theprocess according to the invention and the advantages that it offersmore readily understandable.

EXAMPLE

[0052] The sulfur that is to be treated contains:

[0053] 12% (by mass) of water,

[0054] 3.15% (by mass) of an equimolar mixture of the sodium salts ofnitrilotriacetic and iminodiacetic acids,

[0055] 0.4% (by mass) of iron.

[0056] In a 2 liter double jacket reactor, 600 g of polyethylene glycolof molar mass 400 g/mol (PEG 400) is heated to 125° C. while beingstirred mechanically. 400 g of sulfur to be treated is introduced there.The sulfur melts, and the water evaporates. The stirring is maintainedfor 15 minutes. The stopping of the stirring makes possible thedecanting of the liquid sulfur. The salts appear in the interfacebetween the lower phase, the liquid sulfur and the upper phase, PEG 400.After 2 minutes of decanting, the liquid sulfur is drawn off, and 600 gof PEG 400 is left in the reactor. 400 g of the sulfur to be treated isthen introduced while being stirred mechanically and the process stepsthat are described above are followed again. The addition of sulfur ishalted when the separation of phases is slow and imperfect. The sampledsulfur that is clear yellow in color contains less than 0.1% by weightof carbon, and its ash content is less than 0.1% by weight. Thus, 6400 gof impure sulfur could be treated before it is necessary to regeneratethe solvent that contains the impurities that are solid, dissolvedand/or in suspension.

[0057] The example is repeated by using as organic solvent 1-octanolinstead of PEG 400, and approximately the same results were obtained.

1. Process for the purification of sulfur that contains impurities,characterized in that in a contact zone, solid sulfur is brought intocontact with at least one organic solvent that is selected from thegroup that is formed by monohydric alcohols with 8 to 40 carbon atoms,polyols that comprise 2 to 8 hydroxyl groups and that have 8 to 40carbon atoms, polyalkylene glycols and polyalkylene glycol ethers undersuitable conditions for carrying out the melting of sulfur; the supplyof sulfur is halted, and the mixture that is obtained is allowed todecant so as to produce the segregation or liquid/liquid separation ofthe purified liquid sulfur that is obtained and organic solvent thatcontains impurities, and at least a portion of the purified liquidsulfur is drawn off from the lower portion of the contact zone. 2.Process according to claim 1, wherein at least a portion of the organicsolvent that results from the decanting stage that contains impuritiesis drawn off, said organic solvent is purified so as to remove from itat least a portion of the impurities; at least a portion of the purifiedorganic solvent is collected, and it is recycled in the contact zone. 3.Process according to claim 1, wherein at least a portion of the organicsolvent that results from the decanting stage is drawn off, and it iseliminated.
 4. Process according to one of claims 1 to 3, wherein priorto the stage of bringing it into contact with the organic solvent, astage of separation of at least a portion of a “redox” catalyticsolution mixed with the sulfur to be purified is carried out.
 5. Processaccording to one of claims 1 to 4, wherein the sulfur that is to bepurified contains 2 to 90% by weight of the catalytic solution andpreferably 5 to 50% by weight.
 6. Process according to one of claims 1to 5, wherein the temperature of the contact zone is 120 to 160° C. 7.Process according to one of claims 1 to 6, wherein the sulfur that is tobe purified is brought into contact with the solvent in proportions ofbetween 5 and 80% by weight of sulfur.
 8. Process according to one ofclaims 1 to 7, wherein a vapor phase that is condensed so as to recovera liquid phase that contains impurities and a residual gaseous effluentthat is purified before disposing of it are drawn off.
 9. Processaccording to one of claims 1 to 8, wherein the organic solvent is apolyethylene glycol.